There are conventionally known automatic transmissions with a manual mode, such as, for example, those disclosed in PTLs 1-3. In this type of automatic transmissions, when the manual mode is selected, then if the engine RPM reaches a predetermined upper limit, the transmission automatically shifts up to prevent engine overspeed.
Also, in the manual mode, the transmission can shift down in response to a driver's request. When, however, the transmission shifts down by a driver's downshift operation (an operation of a shift lever), the engine RPM may immediately increase and then the transmission may automatically shift up. In this case, hunting occurs and is annoying to the driver. Therefore, a hysteresis is provided between the RPM of the output shaft at which the transmission automatically shifts up (the output shaft RPM of the automatic transmission) and the RPM of the output shaft at which the transmission is allowed to shift down, thereby preventing the hunting.
Specifically, as shown in FIG. 8, an automatic upshift map defining the output shaft RPM (horizontal axis) and the accelerator opening degree (vertical axis) as parameters is prepared, where an automatic upshift line (solid line) is set in a higher rotational speed region, and a downshift permission line (dashed line) is set in a lower rotational speed region. Thus, the downshift permission line and the automatic upshift line for preventing overspeed are arranged with a predetermined hysteresis being provided therebetween. As a result, the transmission is prevented from automatically shifting up immediately after shifting down in response to a driver's downshift operation, i.e, hunting is prevented from occurring.